Photochromism is the reversible function of a certain compound that it changes its color swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and returns to its original color when it is put in the dark by stopping its exposure to light. A compound having this property is called “photochromic compound” and used as a material for photochromic plastic lenses.
For the photochromic compound used for this purpose, the following properties are required: (A) the degree of coloration at a visible light range before ultraviolet light is applied (to be referred to as “initial coloration” hereinafter) should be low, (B) the degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high, (C) the speed from the time when the application of ultraviolet light is started to the time when the color optical density reaches saturation (to be referred to as “color development sensitivity” hereinafter) should be high, (D) the speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high, (E) the repeat durability of this reversible function should be high, (F) the solubility in a monomer composition which will become a host material after curing of the photochromic compound should be high so that its dispersibility in the host material in use becomes high, and (G) the compound should develop a color of a neutral tint such as brown or gray by itself.
As the photochromic compound which can satisfy these requirements, there are known chromene compounds having an indeno(2,1-f)naphtho(1,2-b)pyran structure represented by the following formula (I) as the basic skeleton (refer to a pamphlet of WO2005/028465, a pamphlet of WO2010/065393, a pamphlet of WO2011/016582 and the publication of US20090309706).

The inventors of the present invention demonstrated that out of these chromene compounds, chromene compounds having a sulfur-containing substituent represented by the following formula (II) are particularly excellent in initial coloration, color optical density, fading speed and developed hue (refer to a pamphlet of WO2011/016582).
(In the formula, Me means a methyl group.)The above pamphlet shows that a compound obtained by substituting a methylthio group at the 7-position of the chromene compound of the formula (II) by a phenylthio group has the same effect.
Although the above compounds are very excellent, when the inventors of the present invention conducted various studies, they found that when an optical article manufactured by using the above compound having a sulfur-containing substituent is kept at a high temperature, it yellows or its developed hue changes at the time of exposure according to the conditions. This suggests that restrictions may be imposed on a production process which requires a high-temperature treatment, or there may occur a problem with storage stability.